Orthogonal frequency division multiplexing (OFDM) has the advantages of resisting frequency selective fading and narrow-band interference. In this paper, we apply the OFDM technique to the design of underwater acoustic communication system, where the cyclic prefix interval is carefully designed to overcome inter-symbol interference caused by underwater multipath propagation. Via Matlab simulations, we show that the OFDM system can efficiently eliminate multipath interference in underwater acoustic communications. Our results confirm that the OFDM-based transmission technologies can support high speed data transmission in the field of underwater communications.